[8.04] Neutron Star Structure in the Presence of Scalar Fields

Motivated by the possible presence of scalar fields on
astrophysical scales, suggested by the apparent acceleration
of the universe implied by the supernovae surveys, we
present models of neutron star structure including the
contribution of a (massless) scalar field to the stress
energy momentum tensor, in addition to that made by the
normal matter. To that end we solve the coupled Einstein --
scalar field -- hydrostatic balance equations to compute the
effect of the presence of the scalar field on the neutron
star structure. We find that the presence of the scalar
field does change the structure of the neutron star,
especially in cases of strong coupling between the scalar
field and the matter density. We present the neutron star
radius as a function of the matter--scalar field coupling
constant for different values of the neutron star central
density. The presence of the scalar field affects both the
maximum neutron star mass and its radius, the latter
increasing with the value of the above coupling constant. We
also compute particle and photon geodesics in the geometry
of these neutron stars as well as to the geometry of black
holes with different values of the scalar field. Our results
may be testable with timing observations of accreting
neutron stars.